Innate immune responses are dictated by a panel of pathogen recognition receptors, downstream signaling from the receptors and the stimulated activities of various effector molecules. IRF8 is known as an interferon (IFN)-responsive transcription factor that plays critical roles in regulating the development of myeloid and dendritic cells and the activity of a number of genes, such as IL-12 and iNOS, involved in innate responses. Much of the activity of IRF8 in vitro was previously shown to require its ability to heterodimerize with PU.1 and possibly other transcription factors to mediate transcriptional activation or repression. An in vivo test of this model was provided by studies of BXH2 mice that identified a point mutation in IRF8 in the domain required for heterodimerization. It was shown that mice bearing this mutation were very similar to those bearing a null mutation of the gene, but that the null and point-mutant mice differed in their patterns of dendritic cell maturation. This indicated that most, but not all in vivo activities of IRF8 are dependent on its ability to dimerize with other transcription factors. [unreadable] Previous studies demonstrated that IRF8 is expressed to varying extents in subsets of dendritic cells (DC) of bone marrow origin. The status of IRF8 expression in follicular dendritic cells (FDC) that are not of bone marrow origin was not known. Studies of human and mouse FDC demonstrated that they expressed little or no IRF8. In addition, it was found also that IRF8 is expressed a much lower levels in tingible body macrophages of germinal centers than in other macrophage subsets. Studies of IRF8 knockout mice showed that the distribution and number of marginal zone macrophages was greatly altered while metallophilic macrophages on the other side of the marginal sinus were unaffected. These findings indicated that IRF8 was differentially expressed and functional in subsets of DC and macrophages.